Re: format difference between a 4040 and a 1541

Hi,

On Sun, Sep 28, 2025 at 2:52 PM André Fachat <afachat_at_gmx.de> wrote:
>
> As far as I remember it is this.
>
> The dual CPU drives write 8 byte of real gap data bytes, which get translated into 10 GCR bytes.
>
> When they converted to the single CPU drives, they forgot that the old drives converted between real and GCR in hardware. So they only wrote 8 bytes too, but which was then GCR bytes. I believe the early 1540/1541 DOS images should have that value. They then later moved to 10 and back to 9 IIRC.
>

Yes, this is consistent with what I can observe on the gap between
HEADER and SECTOR (data), and this appears to be the same for all
clock zones.
In other words, the 4040 seems to make a little longer gap H to S with
respect to a -03 1541.


> As for the gap between end of sector data and beginning of new header sync, IIRC this was calculated at format time where the drive would basically measure the length of the sector in bytes (which depends on the speed of the drive as the bit frequency is fixed) and try to distribute the sectors evenly.

ok this could also explain the difference I'm seeing, I'll try
different 1541 drives then, even if I'm quite sure all of my drives
were set to 300 rpm sometimes in the past 10 years (as soon as they
needed some service anyway).
>

> To move that out was one of the optimizations I did in my (long forgotten) fast format routine...

in reality I might be observing a 1541 with speeddos+ modified ROM and
that has a fast format routine instead of the standard one, I don't
recall what drive I've used to format the floppy I'm taking as
"reference". I'll do more tests (including a 1540 ROM format).

Thanks
Frank